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Is there is only one step the same as the difference is not clear-cut?
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c.p.Jan 25 '14 at 23:17

I put one step because I think is the more appropiate translation for the idiom he is asking (maybe is too poetic). All three idiomas means there is no a clear difference between both words. Look at wordreference's forum, or maybe we can ask on englishstackexchange. Likewise, as it was commented 'delgada linea' seems to be more used.
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AlexBcnJan 28 '14 at 13:56